Tolerance mechanisms are important determinants of cellular resistance to DNA damage. Eukaryotic cells are not only capable of repairing DNA damage caused by radiation or genotoxic chemicals but can also perform translesion synthesis i.e., DNA replication past base damage with reduced or absent pairing properties in the template. Such bypass requires the action of specialized DNA polymerases. In Escherichia coli, the proteins involved in such mechanisms are subject to tight transcriptional controls and are induced as part of a large regulon of damageinducible genes, the SOS-system. In the budding yeast Saccharomyces cerevisiae the DinB/UmuC homologue RAD30 was shown to encode a polymerase (polymerase n) that can bypass ultravioletradiation induced TT-cyclobutane pyrimidine dimers with surprising accuracy - a predominantly errorfree bypass pathway. Rad3Op represents the yeast homolog of the human protein defective in xeroderma pigmentosum group V (XP-V) patient cells that are known to bypass UV photoproducts inefficiently and with altered base insertion specificity. The investigators propose to analyze the details of the transcriptional regulation of the UV-inducible RAD30 gene and its biological significance. To this aim, they will: 1) Analyze preconditions and genetic parameters for transcriptional induction of RAD30 following DNA damage; 2) Identify cis-acting promoter elements and trans-acting transcription factors required for regulation of RAD30; and 3) determine the genetic consequences of uninducible or otherwise altered expression of RAD30 and address the assumed antagonistic role of the error-prone bypass polymerase zeta. Details of the regulation of the various damage-bypass processes in eukaryotic cells are essentially unknown at present but are important for many aspects of human health and environmental toxicology. The proposed studies will, for the first time, address such issues in a eukaryotic model system where an in-depth molecular analysis is highly feasible.